All open channels, whether private or public, and whether constructed on private or public land, shall conform to the design standards and other design requirements contained herein.
(A) Manning Equation. The waterway for channels shall be determined using Manning’s Equation.
Q = AV = A (1.486/n) (R 2/3) (s 1/2), where:
A = waterway area of channel in square feet
Q = discharge in cubic feet per second, cfs
V, R, s and n are explained in § 53.11(A)
(B) Channel Cross Section and Grade.
(1) The required channel cross section and grade are determined by the design capacity, the material in which the channel is to be constructed, and the requirements for maintenance. A minimum depth may be required to provide adequate outlets for subsurface drains, tributary ditches, or streams. The channel grade shall be such that the velocity in the channel is high enough to prevent siltation, but low enough to prevent erosion.
(2) Velocities less than 1.5 feet per second should be avoided because siltation will take place and ultimately reduce the channel cross section. The maximum permissible velocities in vegetal lined channel are shown in Table 4. Developments through which the channel is to be constructed must be considered in the design of the channel section.
(C) Side Slopes. Earthen channel side slopes shall be no steeper than 2 to 1. Flatter slopes may be required to prevent erosion and for ease of maintenance. Where channels will be lined, side slopes shall be no steeper than 1-1/2 to 1 with adequate provisions made for weep holes. Side slopes steeper than 1-1/2 to 1 may be used for lined channels providing that the side lining and structural retaining wall are designed and constructed with provisions for live and dead load surcharge.
(D) Channel stability.
(1) Characteristics of a stable channel are:
(a) It neither aggrades nor degrades beyond tolerable limits.
(b) The channel banks do not erode to the extent that the channel cross section is changed appreciably.
(c) Excessive sediment bars do not develop.
(d) Excessive erosion does not occur around culverts, bridges or elsewhere.
(e) Gullies do not form or enlarge due to the entry of uncontrolled surface flow to the channel.
(2) Channel stability shall be determined for an aged condition and the velocity shall be based on the design flow or the bank full flow, whichever is greater, using “n” values for various channel linings as shown in Table 3. In no case is it necessary to check channel stability for discharges greater than that from a 100 year return period storm.
(3) Channel stability must be checked for conditions immediately after construction. For this stability analysis, the velocity shall be calculated for the expected flow from a ten year return period storm on the watershed, or the bank full flow, whichever is smaller. The “n” value for newly constructed channels in fine grained soils and sands may be determined in accordance with the National Engineering Handbook 5, Supplement B, Soil Conservation Service, and shall not exceed 0.025. The allowable velocity in the newly constructed channel may be increased by a maximum of 20 percent to reflect the effects of vegetation to be established under the following conditions:
(a) The soil and site in which the channel is to be constructed are suitable for rapid establishment and support of erosion controlling vegetation.
(b) Species of erosion controlling vegetation adapted to the area, and proven methods of establishment are shown.
(c) The channel design includes detailed plans for establishment of vegetation on the channel side slopes.
(E) Drainage of Waterways. Vegetated waterways that are subject to low flows of long duration or where wet conditions prevail shall be drained with a tile system or by other means such as paved gutters. Tile lines may outlet through a drop structure at the end of the waterway or through a standard tile outlet.
(F) Establishment of New Drain. When the Council determines it is necessary to establish a new drain, each developer must provide the necessary information and meet the requirements of this ordinance.
(G) Appurtenant Structures.
(1) The design of channels will provide all structures required for the proper functioning of the channel and the laterals thereto and travelways for operation and maintenance. Recessed inlets and structures needed for entry of surface and subsurface flow into channels without significant erosion or degradation shall be included in the design of channel improvements.
(2) The design is also to provide the necessary flood gates, water level control devices, and any other appurtenance affecting the functioning of the channels and the attainment of the purpose for which they are built.
(3) The effect of channel improvements on existing culverts, bridges, buried cables, pipelines and inlet structures for surface and subsurface drainage on the channel being improved and laterals thereto shall be evaluated to determine the need for modification or replacement.
(4) Culverts and bridges which are modified or added as part of channel improvement projects shall meet reasonable standards for the type of structure, and shall have a minimum capacity equal to the design discharge or governmental agency design requirements, whichever is greater.
(H) Disposition of Spoil. Spoil material resulting from clearing, grubbing and channel excavation shall be disposed in such a manner which will:
(1) Minimize overbank wash.
(2) Provide, for the free flow of water between the channel and flood plain unless the valley routing and water surface profile are based on continuous dikes being installed.
(3) Not hinder the development of travelways for maintenance.
(4) Leave the right-of-way in the best condition feasible, consistent with the project purposes, for productive use by the owner.
(5) Improve the aesthetic appearance of the site to the extent feasible.
(6) Be approved by the Illinois Department of Natural Resources, Illinois Environmental Protection Agency, or US Army Corps of Engineers (whichever is applicable) if deposited in the floodway.
(Prior Code, Art. 20, § 20-13)